In many laser and dot matrix printers, and in photostatic copiers, large volumes of paper sheets are fed from a storage tray to the transfer station of the machine where the printing or copying is done. Frequently a stack of sheets is stored in a sheet feeding tray, which is spring loaded or motor driven to cause successive sheets to advance upwardly. Mounted proximate the top of the stack of sheets is a sheet feeder, usually a conveyor or other roller device which grabs the top most sheet in the stack and transfers it into the machine.
Unfortunately, the stack of sheets is often non-uniform in thickness due to the nature of the sheets. For example, sheets containing adhesive labels can be non-uniform in thickness. Even plain paper sheets can be non-uniform in thickness. The sheet feeder in these machines expects to see a flat, uniform stack of sheets. A non-uniform stack causes feed jams, or triggers a false indication of an empty hopper.
Various attempts have been made for leveling sheets. U.S. Pat. No. 2,471,066 (Hesson) discloses a compensator mechanism for uneven thickness sheet feeding for use on a can labeler. A hinged section moves up or down to maintain the top of the stack level. U.S. Pat. No. 2,886,314 (Phelan) discloses a compensator mechanism for uneven thicknesses of paper. The mechanism uses a pair of compensator bars for raising the middle as well as one end of the stack to level the top. U.S. Pat. No. 4,593,895 (Myers et al.) discloses a cash dispensing machine cassette having a stack of bills with their long lower edges supported on a floor when loaded in the cassette. A pair of resilient pushers engage the rear of the stack of bills to slide the bills into operative engagement with an ATM which discharges bills therefrom. U.S. Pat. No. 4,942,435 (Kneisel et al.) discloses an apparatus adapted to be mounted on a tray for supporting a stack of sheets having one marginal region of the stack thicker than other marginal regions.
While others have focused primarily on compensating for non-uniformities at the margins of sheets, a need still exists for a simple and inexpensive means of compensating for non-uniformities which exist in the middle or center of the sheet.